As per Star Wars The Old Republic Encyclopedia, the Dark Temple became a dark side nexus. Dromund Kaas itself haven't been labelled as a dark side nexus in the same source.
The advantages of fighting in a nexus are overrated IMO. Unless an individual takes proper advantage of the nexus by using Sith Sorcery or something; the region itself doesn't transforms a dark sider in to a juggernaut of destruction.
Meetra Surik fought and defeated both Darth Sion and Darth Traya in a region strong in the dark side; Malachor V. Why would she not perform good on Dromund Kaas?
Similarly, if it is assumed that Darth Nyriss had homefield advantage in Dromund Kaas, then same would be true for Lord Scourge as well. Why the latter still lost to her?
It makes no sense to underestimate Darth Nyriss's skills and power.